1. Field of the Invention
This invention relates to production of plastic fuel tanks, and more particularly to a process and system for manufacturing plastic fuel tanks using twin sheet thermoforming.
2. Description of the Related Art
Fuel tanks made of plastic are well known for providing a reservoir of fuel for engines and other fuel consuming devices in land, water and air vehicles. Use of plastic fuel tanks provides a lightweight yet durable fuel tank that is corrosion resistant. In addition, plastic fuel tanks can be economically formed into complex shapes to optimally fit within available space within a vehicle.
Plastic fuel tanks are typically produced with a blow molding technique using high-density polyethylene (HDPE) plastic in monolayer or coextruded form. The plastic typically includes a hydrocarbon barrier to prevent passage of fuel and associated vapors there through. During the blow molding process, hollow cylindrical plastic in a molten or softened state is extruded into a mold. The mold includes an interior hollow area defined by a mold surface. Pressurized fluid is fed into the hollow cylindrical plastic to move the plastic into contact with the mold surface. Once the plastic is formed to the mold surface, the mold is opened and the plastic formed in the shape of a fuel tank is removed.
Additional components are usually added to the fuel tank to create a fuel system. The components may include valves, hoses, pumps, level sensors, structural supports, etc. Typically, in fuel tanks formed by blow molding techniques some of these components are installed inside the fuel tank by cutting service holes in the tank. In addition, some of the components are installed outside the tank requiring additional holes, grooves and/or recesses.
Recent changes in government regulations have reduced the amount of allowable fuel vapor emissions from fuel tanks. One way to reduce allowable fuel vapor emissions is through minimization of breaches in the hydrocarbon barrier. Reduction in the number of holes in the fuel tank through internalization of the fuel system components minimizes breaches. One way to internalize the fuel system components is to insert the components during manufacture of the fuel tank.
In plastic fuel tanks manufactured with the blow molding process however, such internalization is not easily achieved. Accessibility to the interior of the fuel tank during the forming process is through relatively small openings making placement difficult. In addition, fuel system components in an extremely compact form may be required to fit through such openings.
Another technique for creating plastic containers uses twinsheet thermoforming. Twinsheet thermoforming techniques are typically performed with a rotary thermoforming machine. In a rotary thermoforming machine, previously extruded thermoplastic sheets are sequentially indexed through stations during processing. Each thermoplastic sheet is individually loaded into the machine at a first station. The thermoplastic sheet is sequentially indexed to a second and third station for heating to a molten or softened state. A sheet emerging from the third station is conformed to the contour of a first mold. The next sheet emerging from the third station is conformed to a second mold. The first and second molds provide the two halves of the plastic container that may then be joined.
Although twinsheet thermoforming with conventional rotary machines is conducive to internalizing fuel system components within the first and second molds, known inefficiencies make the technique unsuitable for high volume production of plastic fuel tanks.
One inefficiency is a result of the length of processing time. Significant processing time is required to heat the sheets to the desired processing temperature. Heating throughout the thickness of the sheets usually occurs gradually to raise the temperature of the sheets without burning or otherwise damaging the thermoplastic. In addition, lag time associated with sequential processing of each sheet adds to the overall processing time. Another inefficiency is the heat loss, and resulting process variations, in a previously heated thermoplastic sheet while awaiting the heating of a subsequently heated thermoplastic sheet. Accordingly, a process and system is needed that can provide efficient high volume production of plastic fuel tanks while allowing internalization of fuel system components.